Organic protomolecule assembly in igneous minerals

Abstract

CH stretching bands, nu(CH), in the infrared spectrum of single crystals of nominally high purity, of laboratory-grown MgO, and of natural upper mantle olivine, provide an "organic" signature that closely resembles the symmetrical and asymmetrical C--H stretching modes of aliphatic -CH(2) units. The nu(CH) bands indicate that H(2)O and CO(2), dissolved in the matrix of these minerals, converted to form H(2) and chemically reduced C, which in turn formed C--H entities, probably through segregation into defects such as dislocations. Heating causes the C--H bonds to pyrolyze and the nu(CH) bands to disappear, but annealing at 70 degrees C causes them to reappear within a few days or weeks. Modeling dislocations in MgO suggests that the segregation of C can lead to C(x) chains, x = 4, with the terminal C atoms anchored to the MgO matrix by bonding to two O(-). Allowing H(2) to react with such C(x) chains leads to [O(2)C(CH(2))(2)CO(2)] or similar precipitates. It is suggested that such C(x)--H(y)--O(z) entities represent protomolecules from which derive the short-chain carboxylic and dicarboxylic and the medium-chain fatty acids that have been solvent-extracted from crushed MgO and olivine single crystals, respectively. Thus, it appears that the hard, dense matrix of igneous minerals represents a medium in which protomolecular units can be assembled. During weathering of rocks, the protomolecular units turn into complex organic molecules. These processes may have provided stereochemically constrained organics to the early Earth that were crucial to the emergence of life.

Figure 1

Infrared spectrum of a nominally high purity MgO crystal, grown from a CO/CO₂/H₂O-laden melt, in the 2800–4200 cm⁻¹ range covering the H—H, O—H, and C—H stretching frequencies.

Figure 2

C—H stretching bands, ν_CH, in the synthetic MgO and an upper mantle derived olivine single crystal recorded before heating. The strongest ν_CH bands appear to arise from symmetrical and asymmetrical C—H stretching modes of —CH₂-entities with minor ν_CH bands probably because of —CH₃. The ν_CH bands of MgO are broader than those of olivine.

Figure 3

After heating for 14 h to 400°C, the ν_CH bands in MgO vanish nearly completely but reappear during annealing at 70°C over a period of 32 days.

Figure 4

Integrated intensity of the ν_CH bands in the MgO crystal annealed at 70°C and other temperatures, plotted as a function of time. The symbols represent measured data, the lines parabolic fits.

Figure 5

(A) An a_o/2[100] screw dislocation in MgO showing how the (100) plane is displaced by a_o/2 after one turn. The large dark spheres indicate O²⁻ anions, the smaller light spheres Mg²⁺ cations. (B) Projection of two idealized edge dislocations generated as part of a subgrain boundary in MgO by the insertion of two half-planes of Mg²⁺ and O²⁻. The arrows point at rows of Mg²⁺ cations that are under high compressive stress and therefore energetically unfavorable (after Harding et al., ref. 38).

Figure 6

(A) Stack of MgO (100) planes with a vertical a_o/2[100] screw dislocation in the center, viewed at right angle. The screw dislocation is decorated by one [O₂C—C—C—CO₂] unit in the energetically favorable trans configuration with the medium size light gray spheres representing O⁻. (B) Oblique view of an edge dislocation in MgO with one row Mg²⁺ removed and two rows of O²⁻ changed to O⁻ (lighter gray spheres). (C) Same edge dislocation with C₄ units segregated into its core. Because of mismatch with the surrounding MgO matrix, the [O₂C—C—C—CO₂] units would have to adopt the energetically less favorable cis configuration.